U01.16.031 Obstructive Lung Diseases

Obstructive lung diseases are characterized by obstruction of airflow, leading to air trapping within the lungs. This results in increased residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC). A hallmark of these conditions is a reduced FEV1/FVC ratio, identified through pulmonary function tests (PFTs). Chronic hypoxic pulmonary vasoconstriction in these diseases can lead to cor pulmonale. Chronic obstructive pulmonary disease (COPD) encompasses chronic bronchitis and emphysema.

Types of Obstructive Lung Diseases

Chronic Bronchitis (“Blue Bloater”)

Presentation: Wheezing, crackles, cyanosis (due to hypoxemia from shunting), dyspnea, CO2 retention, secondary polycythemia.
Pathology: Hypertrophy and hyperplasia of mucus-secreting glands in the bronchi, leading to a Reid index greater than 50%. This index measures the thickness of the mucosal gland layer relative to the wall thickness between the epithelium and cartilage.
Other: Diagnostic criteria include a productive cough lasting ≥ 3 months per year for more than 2 consecutive years. The diffusing capacity of the lungs for carbon monoxide (DLCO) is usually normal.

Emphysema (“Pink Puffer”)

Presentation: Barrel-shaped chest, exhalation through pursed lips (to increase airway pressure and prevent airway collapse).
Pathology: Enlargement of air spaces, decreased elastic recoil, increased lung compliance, reduced DLCO due to alveolar wall destruction.
- Centriacinar Emphysema: Affects respiratory bronchioles while sparing distal alveoli, commonly associated with smoking, and typically affects the upper lobes.
- Panacinar Emphysema: Involves both respiratory bronchioles and alveoli, associated with α1-antitrypsin deficiency, and more frequently affects the lower lobes.
Other: Chest X-ray findings include increased anteroposterior (AP) diameter, flattened diaphragm, and increased lung field lucency.

Asthma

Presentation: Cough, wheezing, tachypnea, dyspnea, hypoxemia, decreased inspiratory/expiratory ratio, pulsus paradoxus, and mucus plugging.
Pathology: Hyperresponsive bronchi leading to reversible bronchoconstriction. This includes smooth muscle hypertrophy, hyperplasia, Curschmann spirals (whorled mucous plugs from shed epithelium), and Charcot-Leyden crystals (eosinophilic crystals from eosinophil breakdown).
Other: Typically triggered by viral upper respiratory infections (URIs), allergens, or stress. Asthma is a type I hypersensitivity reaction. NSAID-exacerbated respiratory disease involves a combination of COX inhibition (leading to leukotriene overproduction and airway constriction), chronic sinusitis with nasal polyps, and asthma symptoms.

Bronchiectasis

Presentation: Purulent sputum, recurrent infections (commonly Pseudomonas aeruginosa), hemoptysis, digital clubbing.
Pathology: Chronic necrotizing infection or obstruction of bronchi, resulting in permanently dilated airways.
Other: Associated with bronchial obstruction, poor ciliary motility (e.g., smoking, Kartagener syndrome), cystic fibrosis, and allergic bronchopulmonary aspergillosis.

Table: Key Features of Obstructive Lung Diseases

Disease	Presentation	Pathology	Other Features
Chronic Bronchitis	Wheezing, crackles, cyanosis, dyspnea, CO2 retention	Hypertrophy/hyperplasia of mucus-secreting glands, Reid index > 50%	Productive cough ≥ 3 months/year for > 2 years, normal DLCO
Emphysema	Barrel chest, pursed-lip breathing	Enlarged air spaces, decreased recoil, increased compliance	Centriacinar (upper lobes, smoking); Panacinar (lower lobes, α1-antitrypsin deficiency)
Asthma	Cough, wheezing, dyspnea, hypoxemia, mucus plugging	Reversible bronchoconstriction, smooth muscle hypertrophy	Type I hypersensitivity, triggers include URIs, allergens, stress
Bronchiectasis	Purulent sputum, recurrent infections, hemoptysis, clubbing	Chronic infection or obstruction leading to airway dilation	Associated with cystic fibrosis, Kartagener syndrome, smoking

Table: Key Features of Obstructive Lung Diseases

Points to Remember

Obstructive lung diseases are characterized by reduced airflow and air trapping, leading to an increased RV, FRC, and TLC.
The hallmark of obstructive lung diseases on PFTs is a decreased FEV1/FVC ratio.
COPD includes both chronic bronchitis and emphysema, with distinct pathological and clinical features.
Asthma is a reversible obstructive lung disease, often triggered by environmental factors and associated with hyperreactive airways.
Bronchiectasis involves permanent dilation of airways due to chronic infections or obstruction, with associated symptoms like purulent sputum and hemoptysis.

Bibliography

This content provides a structured overview of obstructive lung diseases with key clinical and pathological details that are essential for medical students, particularly those preparing for exams like the USMLE.

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USMLE Step 1 Review Series

Curriculum

U01.16.031 Obstructive Lung Diseases

Types of Obstructive Lung Diseases

Chronic Bronchitis (“Blue Bloater”)

Emphysema (“Pink Puffer”)

Asthma

Bronchiectasis

Table: Key Features of Obstructive Lung Diseases

Points to Remember

Bibliography

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